Method and apparatus for measuring properties of materials or structures

ABSTRACT

Stress wave measuring apparatus includes a Hopkinson bar or pressure bar bundle system comprising an input bar bundle having input bars, and an output bar bundle having output bars  104   a . A brittle test specimen is positioned in between the input and output bar bundles. Means is provided to induce a one dimensional compression force into the bar bundles. Instrumentation is introduced onto the input and output bars for measuring values of shear wave or crack propagation. The specimen may be placed under different stress conditions measured by appropriate instrumentation on the bar bundles. In one embodiment, a rod gauge is glued to a rock massive and arranged to measure wave effects generated in the rock massive. Other arrangements provide amplification of the wave effects prior to measurement by instrumentation on the gauge  500.

This invention relates to improvements in or relating to measuringproperties of materials or structures and is more particularly but notexclusively concerned with measuring the mechanical properties ofbrittle construction materials or the mechanical properties of brittlegeological structures. An exclusive performance of some improvements asdescribed herein is that of measuring the mechanical properties underdynamic and quasi-static loading of a material specimen both as a globalvalue and as value distribution over the specimen cross section.

Mechanical characteristics (given by stress strain curves, the influenceon those curves of temperature and strain rate, fracture energy detailsof deformation mechanisms and so on) have been studied successfully inplastics, pure metals and alloys and the knowledge gained thereby hasprovided a scientific base towards optimum production and practical useof such materials. However, the situation with brittle materials orstructures is much more complicated. From a practical standpoint,conventional test installations only allow the rupture limit of suchmaterials to be measured with a corresponding large scatter ordispersion owing to the influence of surface micro-cracks induced in thematerial or structure under test. There is an ever increasing need toreduce the enormous losses due to earthquake and accident impact loadingand thus there is a need to obtain reliable information about rupturegeneration and development in brittle materials of high importance suchas concrete, reinforced concrete, ceramics, composites and rockmaterials: Here an important point is the measurement of the energyreleased as mechanical wave during the fracture development in thebrittle construction materials or in the rock materials; when the massesand volumes of rocks are very large the energy release during fracturegives origin to seismic waves whose precise measurement remains an openproblem.

It is an object of the present invention to provide apparatus (andmethod) for measuring properties of materials or structures moreparticularly during deformation and fracture which is improved in atleast some respect.

According to one aspect of the present invention there is providedapparatus suitable for measuring mechanical properties ofcharacteristics of a brittle material or structure, said apparatuscomprising a Hopkinson bar or pressure bar system comprising an inputbar bundle and an output bar bundle, each said bundle comprising aplurality of parallel bars each equipped with instrumentation formeasuring values of longitudinal stress wave or crack propagationparameters in a brittle material specimen or structure under testlocated, in use, in between the input and output bar bundles.

Further according to this aspect of the present invention there isprovided a method of measuring mechanical properties or characteristicsof a brittle material or structure, said method comprising placing aHopkinson bar or pressure bar system, comprising an input bar bundle andan output bar bundle, under load, and thus placing a brittle materialspecimen or structure located, in use, in between the input and outputbar bundles, under load, and measuring values of shear wave or shearcrack propagation parameters in the brittle material specimen orstructure from instrumentation provided on a plurality of parallel barsforming each said bar bundle.

Usually, each bar o: the bundle gives the local mechanical properties ofthe part of specimen cross section facing of the bar; while summing themeasurements of the bars of the bundle one obtains the global mechanicalproperties of the material specimen or structure.

Means may be provided for placing the bar bundles and thus the materialspecimen or structure under test in compression in a direction along theaxis of the bar bundles. In one embodiment of the apparatus each bar ofthe bundles is equipped with instrumentation to measure locally thestress-strain relationship of the material specimen or structure undertest in addition to measuring values of absorbed energy during rupturingof the material specimen or structure under test, as well as values ofshear wave propagation with a known component of compression stress in acrack plane, the orientation of a rupture surface.

In a further embodiment of the apparatus, each bar of the bundles may beequipped with means for measuring the velocity of a shear crackpropagation where the material specimen or structure under test isplaced in “pure shear”. “Pure shear” means that there is no normalstress in the plane of shear. In this instance, the bar bundles are usedin an unusual manner for a Hopkinson bar or pressure bar system in thatthey are used as receptors of waves emitted by propagating cracks andthus giving a measure of the energy released by crack propagation.

In a further embodiment of the apparatus, each bar bundle is arranged tomeasure the stress-strain relationship in the material specimen orstructure under test in addition to measuring values of energyabsorption during deformation of said material specimen or structure andthe velocity of shear crack propagation therein in conditions ofcontrolled “hydrostatic” pressure. The term “hydrostatic” as applied inthis instance means that loading of the material specimen or structureunder test is combined from equal stress-hydrostatic components plusexcess load along one direction.

Although the above apparatus is suitable for measuring properties ofbrittle materials, it could also be used for investigating ruptureprocesses in plastic materials at the stage of a neck development.

Additionally, means may be provided for placing the Hopkinson barbundles and material structure under test in tension, each bar beingprovided with instrumentation to measure the stress strain relationshipin a particular region of the material specimen or structure, values ofabsorbed energy during the rupture process and values of longitudinalwave propagation within the material or structure, as is already knownfrom the Paper already presented by the inventors entitled “Study of thetrue tensile stress strain diagram of plain concrete with real sizeaggregate; need for and design of a large Hopkinson bar bundle”appearing in the Journal de Physique IV Colloque C8, supplement auJournal de Physique III, volume 4- September 1994, the entire content ofwhich is hereby included into the present specification by reference.The aspect of the present invention as aforedescribed represents afurther development over and above the arrangement shown in this paper.

The aforementioned paper only mentions placing the Hopkinson bar bundlesin one dimensional tension and there is no mention of instrumentationfor obtaining values of shear wave propagation or velocity measurementof shear crack propagation.

Embodiments of the present invention may provide instrumentation formeasuring the following complex of deformation characteristics ofbrittle materials:

The stress-strain relationship and energy absorption in the materialspecimen or structure under test during the deformation process,including during the falling part of the load, in conditions of simplestress-state (e.g. tension or compression) and in conditions of complexstress-state (e.g. tension—tension, under hydrostatic pressure) andaccurate measurements of propagation velocities of both longitudinal andshear waves in such conditions and therefore have a measure of energyrelease from a propagating fracture which is the basic phenomenon of anearthquake.

The Hopkinson bar system is widely used for the study of mechanicalcharacteristics of materials in conditions of high velocity deformation(see for example the Paper by U.S. Linholm entitled “Some experimentswith the split Hopkinson pressure bar, J. Mech.Phys Solids 1964 volume12 317 to 335). The principal feature of such system is the use of twosufficiently long elastic bars (input and output bars) located onopposite sides of a sample for test and analysis of the deformationprocess through signals of incident, reflected and a transmitted waves.

The aforementioned Paper by the inventors appearing in the Journal dePhysique IV represents a modification of the Hopkinson bar system byreplacing the input and output bars by input and output bar bundles inorder to analyse the rupture process developments in large blocks ofbrittle materials. Each bar of the bar bundle is equipped with a straingauge so that during the process of loading a sample by a wavepropagating along the system, the signals received from the bars of thebar bundles will allow rupture development to be controlled. Thus,information is received from the individual bars (prisms) in accordancewith standard Hopkinson bar methodology to find the instant position ofa crack (or cracks in the case of numerous centres of crack generation).However, the present invention develops the ideas suggested in thisPaper by utilising a Hopkinson or pressure bar bundle system in a new orunusual way in order to evaluate shear wave or shear crack propagationin addition to evaluating other deformation characteristics. Inparticular this aspect is realised by using the bars of the bundles asreceptors of the longitudinal and shear waves released by the fracturepropagating through the material specimen or structure.

Thus the present invention may utilise wave effects arising in thedeformation and fracture processes of brittle materials or structuresfor measuring strength parameters and such important characteristics asvelocities and amplitudes of longitudinal and shear crack propagationboth in quasi-static and dynamic loading. Such measurements may allowthe development of a more detailed realistic model of deformation and offracture processes in materials.

The present invention is also concerned from a second aspect withreceiving and analysing information about stress re-distribution in ageological structure such as a rock massive, which may be required forpredicting earthquakes and rock impacts.

There have been many proposals for measuring earthquake wave effects butin the main the devices for measurement of such wave effects in theground belong to two classes: different kinds of pendulums andpiezoelectric gauges are discussed, for example in a paper by M.Wakabayashi, Design of Earthquake-Resistant Buildings, NY McGraw-Hill1986 page 21 to 33.

However, the possibility of using such gauges for detailed analysis ofthe processes involved in the active zone of stress redistribution andaccumulation of energy is limited by the effects of wave reflections notbeing taken into account by such gauges and by the apparent insufficientsensitivity of such gauges to all types of seismic waves characteristicof an earthquake (longitudinal, shear, surface waves etc.).

U.S. Pat. No. 5,487,298 discloses a device for measuring the intensityof high amplitude strain waves caused by the impact of a projectile orthe detonation of an explosive on armour plate. The device comprises asolid cylindrical bar having a pair of strain gauges located ondiametrically opposite sides of the bar and connected so as to recordlongitudinal strain without bending strain.

Thus, according to a second aspect of the present invention there isprovided measuring apparatus arranged, in use, to measure wave effectsgenerated in a geological structure comprising a rod intended, in use,to be fixed to the surface of the structure to receive a longitudinalwave, and wherein the rod is instrumented with sets of gauges attachedto the rod in different directions to allow division of information intointensities of longitudinal and shear components of arrivedperturbation, to enable measurement of shear wave or shear crackpropagation in said structure.

Further according to this second aspect of the present invention thereis provided a method of measuring values of shear wave or shear crackpropagation in a structure using measuring apparatus comprising a rodinstrumented with sets of gauges attached to the rod in differentdirections arranged in use to measure wave effects generated in saidstructure.

The rod is preferably fixed, for example by glue, to the surface of thestructure (for example rock massive). The longitudinal wave may begenerated either artificially (for example a shock wave from anexplosive charge more particularly in a mine) or generated naturally(for example from shear cracks in the regions of active geologicalzones, producing through the redistribution and accumulation of energyan overloaded layer structure which can cause an earthquake of adangerous level or a phenomenon of a similar nature but on a lesserscale—the so-called “rock impact” in mines with a complex geologicalstructure, when a wave generated from a relatively small but closelysituated source can produce severe damage).

Normally, the rod gauge is instrumented to measure information regardinglongitudinal waves generated in said structure.

Furthermore, the measuring instrumentation of the rod gauge may includesets of gauges (for example electrical resistance strain gauges)attached to the rod in different directions to allow division ofinformation into intensities of longitudinal and shear components ofarrived perturbation; this division of information is important for theanalysis of the nature of earthquake source.

The rod gauge may, in use, be arranged to receive an amplified signal ofwave effects generated in the structure. The signal may be amplified,for example, by means of a reflecting and/or refracting part of thesystem or arrangement.

In order to provide a reflecting system, the measuring gauge may bepositioned, in use, into a tubular hole arranged centrally of agenerally convex, preferably parabolic, surface cut or shaped into thestructure under test.

Alternatively, a refracting amplification system may be providedpreferably including a generally cone shape of material extendingbetween one end of the rod gauge and a suitably shaped surface of thestructure (which surface may be concave or convex depending whether ornot the velocity of sound waves in the cone material is greater than, orless than, the velocity of sound waves in the material of thestructure). The surface of the cone in contact with the shaped or cutsurface of the structure under test may be concave or convex to suit.

The material cone may comprise a liquid as a refracting material insidea rubber or plastic envelope, or the like preferably with a thin wall.

In practice, whether or not the system includes means to amplify thesignal in the structure under test, three measuring gauges may bepositioned mutually at right angles to one another at a location underinvestigation in order to give complete information about a stressedstate created by a wave loading.

Further advantageous features of the present invention will be apparentfrom the following description and drawings.

Embodiments of apparatus and method for measuring mechanical propertiesof brittle materials or structures or wave effects generated in astructure in accordance with the present invention will now bedescribed, by way of example only, with reference to the accompanyingmuch simplified diagrammatic drawings in which:

FIG. 1 shows a known Hopkinson bar bundle system for dynamic tensiontesting of plain concrete;

FIG. 2 shows a further simplified diagrammatic view from one side of theHopkinson bar bundle system, for example as shown in FIG. 1, in whichthe Hopkinson bar bundles are placed in one dimensional tension as isknown from the prior art;

FIG. 3 shows in accordance with a first embodiment of the presentinvention a view similar to FIG. 2 in which the Hopkinson bars and thematerial under test is placed in one dimensional compression in order togenerate a shear wave with a known component of compressive stress in acrack plane;

FIG. 4 shows a view, in accordance with the present invention, similarto FIG. 3 in which the material under test is placed in a condition ofpure shear;

FIG. 5 shows, in accordance with the present invention, a view similarto FIG. 4 in which the material under test is placed in one dimensionalcompression in conditions of controlled hydrostatic pressure;

FIG. 6 shows an embodiment of a rod measuring gauge in accordance withthe present invention arranged to measure particular wave effectsgenerated in a rock massive;

FIGS. 7 and 8 show the same rod gauge arranged to measure different waveeffects generated in the rock massive;

FIG. 9 shows a rod measuring gauge in accordance with the presentinvention in combination with a wave reflecting system;

FIG. 10 shows a rod measuring gauge in accordance with the presentinvention in combination with a wave refracting system, and

FIG. 11 shows an arrangement similar to FIG. 10 but modified to takeinto account the relation between the sound velocity values in rockmassive material under test and an attached rod cone.

FIG. 1 of the drawings shows a known Hopkinson bar bundle system 1arranged to test a concrete specimen 2 in a known manner. Each bar 4 aof the input bar 4 and each bar 3 a of the output bar 3 is equipped withmeasuring gauges in the form of electric resistance strain gauges s′ andthe concrete specimen 2 may be placed in tension by means of thehydraulic actuator 5.

FIG. 2 is a much more simplified plan view from the side of theHopkinson bar bundles 3 and 4 (in the direction of arrow A in FIG. 1)illustrating the effect on the concrete specimen 2. Reference C₁represents the velocity of a longitudinal crack propagation(longitudinal crack development occurs when the opposite sides of theconcrete specimen are pulled apart in a direction perpendicular to therupture plane to form an opening “O” more particularly as shown in FIG.2). The one dimensional tensional force applied to the bar bundles 3 and4 is represented by the arrow T.

The afore-described arrangement is known and the stress-strainrelationship in the concrete specimen is determined by means of the bars3 a,4 a individually instrumented with strain gauges s′ which measurethe incident, reflected and transmitted pulses concerning only theportion of the concrete specimen cross section facing the cross sectionof the pair of bars of the bundle. Thus, a value of absorbed energy canbe obtained and a value of longitudinal wave propagation.

This technique is explained fully in the aforementioned paper of theinventors and thus need not be described in further detail.

FIG. 3 is a simplified view of similar form to FIG. 2 of a Hopkinson baror pressure bar bundle system 101 comprising an input bar bundle 103having input bars 103 a and an output bar bundle 104 having output bars104 a of a similar nature to the Hopkinson bar system. 1 shown in FIG.1. However, instead of the Hopkinson bar bundle system 101 beingequipped with a hydraulic actuator 5 to introduce a tensional force Tinto the Hopkinson bar system, the actuator 5 is replaced by means toinduce a one dimensional quasi-static or impact compression force C intothe Hopkinson bar bundles. Additionally, further instrumentation (notdepicted) is introduced onto the bars 103 a and 104 a thus allowing adetermination of a value of absorbed energy in the specimen 102 undertest during the rupture process and the value of shear wave propagationwith a known component of compressive stress in a crack plane, theorientation of the rupture surface. The velocity of a shear wave C_(s)is marked on FIG. 3.

For the manner in which the specimen 102 and Hopkinson bar bundles 103and 104 can be placed into impact compression see, for example, EuropeanPatent Applications EPA-848241, EP-A-849583 and EPA-848264.

FIG. 4 shows a different scenario for measuring the velocity of shearcrack propagation in a modified Hopkinson bar bundle installation 201for a pure shear. Pure shear means that there is no normal stress in theplane of shear. C_(s) represents the velocity of a shear wave in thespecimen 202 under test. Here, the bar bundles 203 and 204 are used inan unusual way for a Hopkinson bar system since they are receptors ofwaves emitted by propagating cracks in the test specimen and appropriateinstrumentation (not depicted) is provided on the bars 203 a,204 a tomeasure the velocity of the shear crack propagation.

FIG. 5 shows a Hopkinson bar bundle system 301 arranged to measure thestress strain relationship in a specimen under test 302 and the value ofenergy absorption during all the deformation processes and velocity ofshear crack propagation in conditions of controlled hydrostaticpressure. Hydrostatic pressure means that loading is combined from equalstress hydrostatic components and excess load along one direction. Withbars 303 a and 304 a produced from high strength steel it is possible toreach a level of hydrostatic pressure of about 1 to 2 Gpa which is of areasonable geological level.

Whilst it is envisaged that the specimens 102,202 and 302 under test areof a brittle material, the Hopkinson bar bundle systems 101,201 and 301could alternatively be used for investigating rupture processes inplastic materials at the stage of a neck development.

Thus, each bar bundle 103,104,203,204,303,304 comprises a plurality ofparallel bars 103 a,104 a,203 a,204 a, 303 a,304 a equipped withinstrumentation for measuring values of shear wave or shear crackpropagation in a brittle material 102,202,302 under test located, inuse, in between the input and output bar bundles 103,104;203,204;303,304.

Additionally, of course, any of the Hopkinson bar systems 101,102,103could be arranged to place the specimen under test in tension as in theprior art arrangement (FIGS. 1 and 2).

Thus, the present invention may facilitate an advanced study of thedeformation process in brittle materials using the basis of wavemechanics principles. The aforedescribed Hopkinson bar bundle systems101,201,301 may enable an assimilation of a complex of essentialdeformation characteristics of brittle materials, such asforce-displacement relationships, including the falling part of theload, value of absorbed energy, values of velocity of longitudinal andshear waves, at different stress rates and therefore of energy releaseas stress waves during fracture propagation and both in quasistatic anddynamic loading, allowing the development of a detailed realistic modelof fracture processes and materials.

FIG. 6 shows a simple rod gauge 500 which is connected, for example bygluing, to the surface r₁ of a rock massive R with the axis 501 of therod measuring gauge extending normal to the surface r₁, said gauge beingarranged to measure wave effects generated in the rock massive. Thegauge 500 is arranged so that it will receive, from the longitudinalwave, falling at the surface r₁ with a particle velocity U_(lx), asignal corresponding to the double particle velocity 2 U_(lX) due to thewave mechanics rule for reflection from a free surface. C_(lx) indicatesthe velocity of the longitudinal wave. U_(lx) represents thecorresponding particle velocity beyond the front of the longitudinalwave. The rod 500 may be provided with measuring instrumentation 502connected by leads 502 a to analysing equipment (not shown) to measureparameters relating to the longitudinal wave.

Similarly, FIGS. 7 and 8 show rod gauges 600 and 700 glued to thesurface r₁ of the rock massive R, provided with measuringinstrumentation 602,702 with leads 602 a,702 a connecting theinstrumentation to analysing equipment (not shown).

FIG. 7 illustrates how (in similar fashion to FIG. 6) the doubleparticle velocity 2U_(sy) can be measured for a shear wave, propagatingin a direction normal to the free surface. C_(sx) and U_(sy) meansrespectively the velocity of a shear wave in the x direction and U_(sy)means a corresponding particle velocity perpendicular to the front ofthe shear wave; the capability of measuring U_(sy) is given by theorientation at 90° with respect to the rod axis, of the strain gauge 602a. However, a shear wave propagating along the free surface r₁ where agauge 700 is glued will cause movement of the free surface with theparticle velocity U_(sx). Rod gauge 700 can be employed to measure thisvalue (not doubled) by strain gauge 702 in FIG. 8. Of course, the threerod gauges 500,600 and 700 could be replaced by a single rod gaugeinstrumented with the instrumentation 502,602,702 to measure all threeparameters discussed in relation to FIGS. 6,7 and 8.

The measuring instrumentation on the rod gauge may thus include a set ofelectrical resistance strain gauges glued on the rod in differentdirections to allow division of information into intensities oflongitudinal and shear components of arrived pertubation. A measuringinstrument oriented along the rod will provide information regardingamplitude and impulse duration of a longitudinal wave. A pair of gaugesarranged at two diametrically opposed positions oriented perpendicularto the longitudinal axis of the rod will provide information regardingthe amplitude of shear waves with corresponding directions of particlevelocity. U_(sy) indicates particle velocity in a shear wave withdirection being perpendicular to the velocity of shear wave propagation.

In order to increase the sensitivity of the measuring gauge for waveanalysis in geological structures, for example in order to receivereliable information about rupture development either in a possiblefuture epicentre of a strong earthquake or in a centre of energyaccumulation so rock massive of a mine with a complex geologicalstructure inclined to a rock impact formation, the arrangement shown inFIG. 9 could be adopted. A distance from the rod gauge 800 to a probablecrack may be about 50 to 100 km, in the case of an earthquake epicentreor about 1 to 5 km in the case of stress redistribution in a mine due toremoval of some volumes of ore and industrial explosions. The amplitudeof expected signals is in proportional to the distance. Thus, anamplitude can be very small, at the level of a background noise fixed bystandard earthquake gauges and therefore the sensitivity of themeasuring apparatus device may need to be higher than that provided forin FIGS. 6 to 8. In order to increase the sensitivity of the apparatus,for example, as shown in FIG. 9 an analogy is adopted between opticalwaves and sound waves. For example, the amplitude of a signal may beincreased essentially by a reflecting system (FIG. 9) or a refractingsystem (SEE FIGS. 10 and 11). With reference to FIG. 9, by the fluxconservation law, the particle velocity of a signal at focal plane couldbe estimated as U=U_(O)S/s where U_(O) is a particle velocity in afalling wave at the entrance section S and s is the section of “animage” that is a cross section of the rod gauge where a signal U ismeasured.

FIG. 9 shows a rock massive R cut to a shape shown in the FIGURE withthe rod gauge 800 inserted into a tubular hole R₁, arranged centrally ofa parabolic shaped rock surface R₂. A reflecting system with a parabolicsurface R₂ is easier to fabricate than a refracting system but ananalysis of the measured signal from the instrumentation gauges 802(having leads 802 a to analysing equipment—not shown) will be morecomplicated due to the splitting of the falling longitudinal wave at thereflection from the inclined surface for longitudinal and shear waves.As should be evident from FIG. 3, waves W travelling in the rock massiveR are reflected by the paraboloid surface R₂ onto the rod gauge 800, thereflection and the successive focussing effect thus amplifying thesignal.

FIGS. 10 and 11 show alternative arrangements X,Y for measuring signalsamplified by refracting systems.

In FIGS. 10 and 11 a cone of material 901,1001, is positioned in betweenone end of the rod gauge 900,1000 and a concave rock massive surface 902of rock massive 903 (FIG. 10) or a convex rock massive surface 1002 ofrock massive 1003. The use of a particular one of the two arrangementsX,Y shown in FIGS. 100and 11 will depend on the relationship between thesound wave velocities C₁,C₂ in the rock massive material and in thematerial of the respective cone 901,1001 extending in between theconcave or convex surface 902,1002 and the end of the rod 900 or 1000.

The focal distance of a spherical surface 902,1002 (spherical inapproximation of small angles) is

F=R′C ₁/(C ₂−C ₁).

Thus, the arrangement shown in FIG. 10 represents the situation wherethe velocity of wave C₁ in the rock material is less than the velocityof the wave C₂ in the material cone 901. R′ is the radius of thespherical surface.

However, when the velocity of sound wave is greater in the rock materialthan in the material cone (i.e. C₁ is greater than C₂) the arrangementshown in FIG. 11 needs to be adopted. Thus, when C₂ is greater than C₁ aconcave rock surface 902 mating with a convex cone surface is requiredas in FIG. 10. When C₂ is less than C₁ a convex rock surface 1002 isrequired to mate with a convex cone surface shown in FIG. 11.

In each case, the waves W in the rock massive 903,1003 will be amplifiedwhen refracted in the cone material 901,902 thus transmitting amplifiedsignals to the rod gauge 900,1000 equipped with instrumentation 904,1004with leads 904 a,1004 a to analysing equipment (not shown).

The material cone 901,1001 may comprise a liquid as the refractingmaterial inside a thin walled rubber or plastic envelope or the like.Such a scenario facilitates good transmission contact at the convex orconcave transmission surface 902,901 a;1002,1001 a and use of the liquidmay allow transmission of virtually only the longitudinal wave in orderto measure accurately the given component of a normal stress.

Three normally positioned gauges may be required to give totalinformation about a stress state, created by a wave loading, in a pointunder investigation. It is possible to avoid practically a reflectedsignal from the far end of a rather short measuring rod if the liquid isused with an appropriate absorption coefficient.

Overall, therefore, referring to FIGS. 9 to 11, the measuringinstrumentation on the rod gauges 900,1000 will measure an amplitudemagnified compared with the value of the particle velocity in thefalling wave (this signal can be recalculated into displacement andpressure, which is a product of density and values of the wavepropagation and particle velocities). Even in the case of purelongitudinally falling waves, after reflection from the inclined surface(see FIG. 9) or refraction at the inclined boundary (see FIGS. 10 and11) a splitting into two waves occurs (due to inclined direction of adisplacement), longitudinal and shear waves travelling with their ownvelocities. Thus, the correct restoration of the amplitude of a fallingwave from the received signal will demand accurate mathematical analysis(it is possible to solve this problem for a fixed geometry and knownwave velocities). In order to avoid these complications in analysis inFIGS. 10 and 11, the second refracting medium may be a liquid aspreviously explained. Shear waves in liquids decay very quickly, thusallowing accurate measurements of the amplitude of longitudinal waves inthe rock massive.

When some rupture processes happen in an active zone of rock massive,waves will be generated. Amplitude of fixed signals in threesufficiently remote gauges together with measured delay times permit afinding of the location of the source and an estimate of the intensityof the rupture. Investigation of the history of the rupture process inthe zone under study is a fundamental scientific way for dangerous rockimpacts and earthquake prediction.

Thus, FIGS. 6 to 11 provide means for recieving, with help stress gaugesbased on waves mechanics principles, information about crack propagationprocess in a rock massive which gives a base for an analysis of stressredistribution and energy accumulation in a probable source of adangerous earthquake or in the probable source of a rock impact.

The above embodiments are described by way of example only. Variationsare possible without departing from the invention.

What is claimed is:
 1. Apparatus (101) specially designed for measuring mechanical properties or characteristics of a brittle material or structure (102), said apparatus (101) comprising a Hopkinson bar or pressure bar system (103,104) comprising an input bar bundle (103) and an output bar bundle (104), each said bundle comprising a plurality of parallel bars (103 a,104 a) each equipped with instrumentation for measuring values of shear wave or shear crack propagation in said brittle material or structure under test located in between said input and output bar bundles (103,104).
 2. Apparatus (101) as claimed in claim 1 having means for placing said bar bundles (103,104) and thus the material structure under test in compression in a direction along a longitudinal axis of the bar bundles.
 3. Apparatus (101) as claimed in claim 1 or claim 2 in which each bar is equipped with instrumentation (s′) to measure the stress-stain relationship in a particular region of the material or structure under test, in addition to measuring values of absorbed energy during rupturing of the material or structure under test, as well as values of shear wave propagation with a known component of compression stress in a crack plane.
 4. Apparatus (201) as claimed in any of claims 1 or 2 having means for placing the Hopkinson bar bundles (103,104) and material structure (101) under test in tension.
 5. Apparatus (201) as claimed in claims 1 or 2 in which each bar (203 a,204 a) is equipped with means for measuring the velocity of a shear crack propagation where the material or structure under test is placed in pure shear.
 6. Apparatus as claimed in claim 1 including instrumentation for measuring the following complex of deformation characteristics of brittle materials: the stress-strain relationship and energy absorption in the material or structure under test during the deformation process; including during the falling part of the load, in conditions of simple stress-state (including tension or compression) and in conditions of complex stress-state (including tension-tension under hydrostatic pressure) and accurate measurements of propagation velocities of both longitudinal and shear waves in such conditions.
 7. Apparatus (101) as claimed in any one of claims 1 , 2 or 6 in which three measuring gauges are positioned mutually at right angles to one another at a location under investigation.
 8. A method of measuring mechanical properties of characteristics of a brittle material or structure (102), said method comprising the steps of: placing a Hopkinson bar or pressure bar system (103,104), including an input bar bundle (103) and an output bar bundle (104), under load, placing said brittle material or structure (102) located in between said input and output bar bundles (103,104), under load, and measuring values of shear wave or shear crack propagation in the brittle material or structure (103,104) from instrumentation provided on a plurality of parallel bars (103 a,104 a) forming each said bar bundle (103,104).
 9. A method as claimed in claim 8 wherein the bar bundles (103,104) and brittle material or structure (102) are placed in compression.
 10. A method as claimed in claim 8 wherein the brittle material or structure (202) is placed in “pure shear”.
 11. A method as claimed in claim 10 wherein the bars (203 a,204 a) are used as receptors of stress waves.
 12. A method as claimed in claim 8 wherein the brittle material or structure (302) is placed under hydrostatic pressure.
 13. Measuring apparatus arranged to measure wave effects generated in a geological structure (R), comprising a rod (600) adapted to be fixed to a surface (r₁) of said geological structure to receive a longitudinal wave, and wherein said rod (500) is instrumented with sets of gauges (502,602,702) attached to the rod (700) in different directions to allow division of information into intensities of longitudinal and shear components of arrived perturbation, to enable measurement of shear wave or shear crack propagation in said structure.
 14. Apparatus (900,1000) as claimed in claim 13 having a refracting amplification system including a generally cone shape of material (901,1001) extending, in use, between one end of the rod (900,1000) and a suitably shaped surface of the structure (R).
 15. Apparatus (500,600,700,800,900,1,000) as claimed in either claim 13 or 14 in which three measuring gauges (500,800) are positioned mutually at right angles to one another at a location under investigation.
 16. A method of measuring values of shear wave or shear crack propagation in a structure (R) using measuring apparatus (800) comprising a rod (800) instrumented with sets of gauges attached to the rod in different directions arranged to measure wave effects generated in said structure (R).
 17. A method as claimed in claim 16 including the step of arranging the rod (800) to receive an amplified signal of wave effects generated in the structure using a reflecting or refracting system.
 18. A method as claimed in claim 17 including the step of positioning the rod (800) into a tubular hole (R1) arranged centrally of a convex surface (R2) cut or shaped into the structure (R) under test.
 19. A method as claimed in claim 17 including the step of providing a generally cone shape of material (901,1001) and placing this between one end of the rod (900,1000) and a suitably shaped surface of the structure (R).
 20. A method as claimed in claim 19 in which the structure surface (902) is concave and the velocity of sound waves (C₂) in the cone material (901) is greater than the velocity of sound waves (C₁) in the material of the structure (R).
 21. A method as claimed in claim 19 in which said structure surface (1002) is convex and the velocity of sound waves (C₂) in the cone material (1001) is less than the velocity of sound waves (C₁) in the material of the structure (R).
 22. A method as claimed in any one of claims 19 to 21 in which the material cone (901,1001) comprises a liquid as a refracting material inside a rubber or plastic envelope, preferably with a thin wall. 